-cos(π/16)sin(π/16)cos(π/8)
＝(-1/2)·2cos(π/16)sin(π/16)cos(π/8)
＝(-1/2)·sin(π/8)cos(π/8)
＝(-1/4)·2sin(π/8)cos(π/8)
＝(-1/4)·sin(π/4)
＝(-1/4)·(√2/2)
＝-√2/8.